You are not starting out with an equation, since you have no number on the right side, just a ?. Therefore x could be anything

If you are supposed to solve the equation
2x^2 -11 x -21 = 0,
then factor it into
(2x +3)(x-7) = 0
That tells you that the answers are x = -3/2 and x = 7.
If you are trying to use the quadratic formula
x = [-b +/- sqrt(b^2-4ac)]/2a
to get the answers, then b = -11 and c = -21. You got the signs wrong. I have no idea what your a*c product and p are supposed to sigfnify.